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So I started toying around (read: developing full-time) with Xbox programming using XNA GS 3.0. In fact I took a big Windows Game project with many satellite Game Libraries, a Content Pipeline Extension, a content editor, an automatic serialization library… and “ported” most of it to Xbox. But since the editor will remain Windows-based, the engine and most of the code needs to stay cross-platform, compatible with both Windows and Xbox.

And I hit a few walls.

I feel like these are things many people working with XNA will encounter. XNA’s been around for a while now, since version 1.0. Many of these things are already widely discussed in the blogosphere and forums. Also, I’m aware that GS 3.1 is around the corner and it’ll address at least the first point of my rant…

Still, here’s a handful of things that surprised or annoyed me in the transition :

Suppose you have a pretty big project that has custom datatypes, and those datatypes are compiled to XNB files using custom ContentTypeWriters and then read back using ContentTypeReaders. You usually need a Content Pipeline Extension project for that, and this project would reference your Engine or whatever project owns the datatypes that you want to compile.

Before very recently, I never quite understood why all official samples had the Reader classes in the Game project, while the Processors, Writers and Importers were all in the Content Pipeline Extension project. Why decouple it like that, and why join them with a fully-qualified assembly string in the GetRuntimeReader method of Writer classes? Moreover, putting Readers in my Extension project always worked in my Windows-only solutions, and it all felt nice and clean.

But when doing everything for Xbox and Windows, the reason becomes clear…

The Content Pipeline Extension project is a standard C# project in XNA GS 3.0. Not a “Game Library” project or any other special container. This means that it won’t be duplicated if you do an Xbox version, and it makes sense; you only need to compile content on your Windows machine.

So your Content Pipeline Extension needs to have a reference to your content datatypes, in some Game Library project. Since the Extension is for Windows only, it’d reference the Windows version of that Game Library. And then if your Readers are in the Extension, your Xbox game needs to reference it to load assets… which means the Xbox and Windows versions of your data structure Library project would coexist on the Xbox. This can’t work!

Besides, the Xbox project doesn’t need to access Processors, Importers and Writers. All it needs to be able to read content and then use it. These other content pipeline classes may even use Windows-specific assemblies like GDI+, why not? They’re certainly useful for image processing.

So bottom line, keep Readers in your Game project if it’s a small project, or in a Game Library for both Xbox and Windows. And if your Content Pipeline can reference this Readers-container, then no need for a hardcoded String for GetRuntimeReader, you can just get the assembly-qualified-name from Reflection classes!

2. The Xbox doesn’t like garbage

The first thing I noticed after I got the game running were hiccups in the framerate, every two seconds or so. But the framerate apart from that was a constant 60. I half-expected this,… it’s the garbage collection.

This paper by three people at the FZI Research Center for Information Technology explains it much better than I can, so I’ll just quote them… :

The .NET Framework on PC uses a generational approach, making garbage collections less painful and more resilient to large numbers of objects. With 512 MiB of GDDR3 memory shared between GPU and CPU the Xbox 360 garbage collector can’t afford such luxury.
The garbage collection of the .NET Compact Framework for the Xbox 360 always has to check all objects. Therefore the time a collection takes increases linearly with the number of objects. Further a collection will be triggered whenever 1 MiB of memory has been allocated.

This means you really need to stop carelessly allocating to the heap when doing an Xbox game. There are several “known causes” of heap garbage with the XNA Framework, but it’s easy to start going on a witch-hunt and replacing all foreach(in) statements by plain for(;;) or stuff like that… It’s a much better idea to find out what are the bottlenecks in your application and fix them starting by the bigger ones. You’ll probably end up solving most of the jittering without making your code look like C.

Update : See this post for more information on typical causes of heap garbage.

3. The Compact .NET Framework needs your help

The Xbox .NET implementation is not the full-blown framework, it’s based on a subset called the Compact Framework, which is also used on mobile devices and embedded systems. This comes at a small cost : you have to complete it to fit your needs.

It’s actually pretty cool that LINQ is supported and all 3.0 features work flawlessly. But here’s a short list (off the top of my head) of things I found missing, some important, some easily worked around, all of them at least mildly annoying… :

ParameterizedThreadStart : You can’t start a Thread with a context object in the CF. You then need a shared context object in the parent class.

Type.GetInterface(string, [bool]) : You can’t query the interface of a type via reflection in the CF… at least not a single one by name. The GetInterfaces() method is supported, so might as well just use that.

Math.Log(double, double) : Actually, there is a Log(double) function, but it’s with the natural base. The custom-base one is not supported. Seriously? (and I know, the workaround is a one-liner)

HashSet : I love the .NET 3.5 HashSet generic class. It’s really complete, super fast… but the CF doesn’t have it. I ended up faking one with a Dictionary as a backing collection, and rewriting the set operators (UnionWith, IntersectWith, etc.) that I really used.

4. You’ll pretty much need a Content project

I don’t like the fact that in a standard XNA project, the content is compiled at build-time, in the Visual Studio IDE. For many reasons… one of them being that I’m not the one producing the content, the artist does, and he certainly doesn’t want to have Visual Studio installed. Another one being that my Content Processors are super heavy and VS sometimes crashes with a OutOfMemoryError before the build completes.

So what I did is write a content compiling tool that uses the MSBuild API to generate something like the .contentproj (yet simpler) based on the filesystem automatically, and compile it externally without needing Visual Studio. This works really great for Windows, we’ve been using it for months now.

But for Xbox… the deployment process is also tied to Visual Studio. And I’m not expert enough at MSBuild technologies to be able to replicate deployment outside of it. So I ended up making a content project only for the Xbox version of my Game project, and compile it separately when I need to test on XNA Game Studio Connect. This works OK, but I’m still a little unhappy about this whole Visual Studio dependency. I hope they look into it properly in the future.

So I was under the impression that you needed to have a content project for Xbox deployment, but Leaf (first comment) pointed out two ways of handling content compilation outside of Visual Studio. I’m definitely going to use the first one!

And…

These are the big points for now. Stuff I thought about adding : RenderTargets act different on Xbox and PC (but Shawn Hargreaves already blogged extensively on the subject and it’s way better than it used to be), Edit-And-Continue is not supported when debugging on the Xbox (but that would’ve been asking for the moon!), you get different warnings for shader compilation when targeting the Xbox360 platform so you should pay attention to that, etc. etc.

I’m still very much halfway through the conversion process, and I’m still learning, and still discovering oddities. If you have advice or corrections, please let me know through comments! On my part I’ll keep this post updated if I hit another big wall.

A faster PersistentThread

In my last post, I wrote a class that keeps a single thread alive in order to re-use it without having to create threads over and over, an operation that sounded like a lot of overhead when you have over 30 updates per second (i.e. in a game loop). However, the benchmarks showed that it was actually slower to use synchronization primitives than to just recreate the threads every time!

It turns out that using a Monitor for the wait/signal operations wasn’t so good. There are lighter primitives in .NET that can be used for that simple operation, where all you need is a boolean semaphore. After researching a little bit, the best object for the job is the ManualResetEvent. This allows you to put a thread in sleep mode with the WaitOne() method, and wake it from another thread using the Set() method.

I ended up using twoManualResetEvents : one for a thread waiting to be started with a new work unit, another to simulate a thread “Join” operation (without actually killing the thread). And the cost of using them is MUCH smaller than Monitors! Here are the new performance numbers :

Seeing as the iterative version takes ­~22.33 seconds to execute, the theoretical minimum time it could take on my Core 2 Duo is ~11.17s. And the new PersistentThread takes only ~0.16s less than that!
Also notice that the former speed champion, the base-line ParameterizedThreadStart method, is now slightly slower than the new PersistentThread. Honestly, I don’t think one can do any better with just two threads.

Task Parallel Library

The fancy lads at Microsoft have been working hard lately on bringing new ways of working with concurrent code. It was one of the big subjects on Channel9 this autumn.

So they released a CTP back in June of the Parallel Extensions for .NET, which features the Task Parallel Library and a front-end for using it with the Parallel static class and Parallel LINQ. I love it, and I can’t wait to use the official, final release in .NET 4.0. But for now, the CTP is an excellent reference point to see what kind of performance and ease of integration the TPL brings to your project.

The simplicity of my test codepath for the TPL boggles the mind, compared to all the other methods I’ve tested.

The number of threads used, how the data is partitioned, and the actual thread/context handling is all abstracted to its simplest form. It really makes C# 3.0 lambda expressions shine.

So, it’s cute alright. But how does it perform? Actually, the TPL will allocate as many threads as it thinks would be beneficial, like an automatic thread pool, based on how many processors you have and the workload you give it. I’m not sure of the details, but that’s how I understood it. So I suspect that it uses more than two threads if doing so will give more processor time to your code; this is relevant when there are many other threads waiting for the processor, and more of these threads being your code = more chance of it being executed.

It’s definitely fast, but not as fast as my new PersistentThread! But it’s so much more flexible, easy to use and scalable that it’s the obvious choice for concurrency in .NET,… when it’ll be officially released. :)

A final note on the Parallel Extensions, there’s a new class called ManualResetEventSlim in this CTP which suggests that it’s more optimized or leaner. I didn’t use it because it depends on libraries that cannot be shipped, but quick test showed that the performance was more or less the same as ManualResetEvent.

Vista > XP

Since the last post, I upgraded (though some might say downgraded…) to Windows Vista. The experience isn’t totally smooth since my laptop is being dumb, but one of the upsides of upgrading is having access to new kernel code that appears to be more efficient.

My last benchmarks had quite a margin between the different tests (generic context & delegate, class-local context, etc.). In Vista, it’s pretty much the same :

Those are really insignificant differences. I suspect that either calling delegates, creating objects, or just casting objects has become faster in Vista. It’s really hard to tell, but the good news is that you can really choose whatever approach you like best : they’re all equally as fast.

Two threads per core?

A final test I wanted to share is to double the workload on an Idle processor and see what happens. So instead of creating a single PersistentThread, I created 3, which means 4 active threads including the current one. The data is split in four equal parts, and…

… and I don’t know what to say. The results are ridiculously unstable, and the code is just confusing and ugly. I wouldn’t recommend it.

Also, I did consider using the .NET 2.0 ThreadPool static class, but it seemed a little confusing to use, especially if I want to join the threads after everyone’s done his work. I think I can wait for the TPL for a proper solution. ;)

Updated test project

Here’s the updated project, now you’ll need the Parallel Extensions CTP for it to compile!LoopParallelismUpdate.zip (150kb), Visual Studio 2008 Solution for C# 3.5.

Super HYPERCUBE (capitalization may vary) is a game I made with the fine folks at Kokoromi for this year’s (2008) Gamma art/game show in Montréal. Gamma is a themed game party that’s been happening for three years now, each year with a design constraint for all the games that apply; this year’s Gamma 3D was about red/cyan stereoscopy aka color anaglyphs.

SHC Logo/Splash Screen : It’s actually all 3D and animated.

The idea is that you have to fit a cluster of cubes inside a wall that represents a projection of that cube on one of its faces, with a series of rotations applied. So it’s a bit like the Japanese “Human Tetris” game shows, which is the comparison that our recent blogcoverage have been using, and it’s exactly right. Except you’re handling a random cluster of cubes.

Development Timeline : From concept art, to Sketchup mockup, to early prototype, to final product!

The game, like all other Gamma games, was made to be easy to learn and fun within 5 minutes, because it was to be played by the public and we want to get as many people to play as possible. So the concept is fairly simple, but I was surprised about how competitive the gameplay was on the showfloor! Until the last minute, I had a fight with a fellow party-goer for the #1 high score, which I won by an unfair margin, which I assume was due to luck and… well… hours of testing the game while making it. ;)

Good Luck With That : The shapes get pretty crazy in the last moments.

But our game’s most awesome feature is not just stereoscopy, it’s wiimote headtracking! Which is a bummer, because even if the game is now available for download, I assume noone will have the setup to play it as it was meant to be played. (The most important part being IR-LED-mounted glasses!)

You can still use an Xbox 360 gamepad or just the keyboard to play it, and that’s how I’ve been testing it most of the time. It’s just nowhere as immersive without the headtracking… the combination of that and stereoscopy worked really well for us. There will probably be videos of people playing at Gamma 3D sometime soon, I’ll update this post with links.

The game itself was programmed using C# 3.5, the Truevision3D 6.5 engine and part of the XNA framework (I’ve bundled the DLL) for full Xbox controller support. There is no sound, this is voluntary… there was a DJ at the actual event. :)

Chances are that if you’re reading this blog, you’ve already heard of this, but I’ll put it here for archiving. :)

Trouble In Euclidea is a game I made for the TIGSource Bootleg Demake Competition as an obvious demake of Geometry Wars. My intent was to demake the graphics (by using ASCII art for everything) more than the gameplay, but it turns out most people did both, which makes much more sense as the gameplay ends up being more original and it plays less like a cheap clone. Mine kinda does. :P

I ended up with five votes in the competition, which places my game 26th out of 30 positions. It may not sound like much, but I’m really happy that I got votes at all! There are really amazing entries that scored around mine, SHADE: Ghost Academy and DamN for instance…

It was also a nice experiment in fast prototyping. I did the game in a single month, but only on weekends except for the last week. Which means I spent at most 15 days on the game, from start to finish, from graphics to game code.

I used C# 3.5, IrrKlang.NET, TV3D and my spiffy new XNA-inspired Component Framework to build it. It worked really well for me, so I decided to release the source of the whole project. As with most of my code recently, it has an almost complete absence of comments, but should be fairly self-explanatory.

(sorry if you got this twice in your RSS, I hit the “publish” button too early…)

A while ago I decided to make an automatic serializer that works just like the XmlSerializer but for the SDL file format, since I like the simplicity and elegance of this data language. The XmlSerializer also doesn’t work natively with Dictionary objects, and crashes when used with certain visibility combinations and C# 3.0 auto-implemented properties.

Making a serializer for any language implies heavy use of reflection to determine the structure of what you’re reading or writing to or from a data file, but also to invoke the getter/setter of the fields you’re serializing.

Performance considerations

Some reflection operations come at a heavy performance cost. Not all of them though! This 2005 article in MSDN Magazine explains that fetching custom attributes, FieldInfo/PropertyInfo objects, invoking functions/properties and members and creating new instances are the costliest operations. Well that’s a problem, because all of those will be handy when writing our serializer.

The same article continues by showing which are the slowest method invocation techniques. The speediest technique are direct delegate use, virtual method calls or direct calls, but those are impossible to use if all you’ve got is a Type and an Object. The next best thing is using a DynamicMethod object, IL emission and a delegate. Having never used IL before, I didn’t grasp all of that, but thankfully there are many other resources concerning the use of DynamicMethod out there.

A post on Haibo Luo’s blog from 2005 makes a performance comparison between Activator.CreateInstance() (by the way, doing “new T()” with a generic type parameter that’s constrained as “new()” is the exact same as calling this method) and various other techniques including DynamicMethod and using it as a delegate. This last technique blows the rest out of the water in terms of speed.

This GPL library on CodeProject written by Alessandro Febretti provides an excellent dynamic method factory. And this other article on CodeProject goes a bit further and shows how to set/get values on fields, and isolates the boxing in helper functions.

What I ended up doing is taking from all of these examples, correcting the problems outlined in the comments of both CodeProject samples, and I built a IReflectionProvider interface that publishes all these costly operations and which can be implemented three different ways :

DirectReflector : Simply via reflection

EmitReflector : With IL emission but no caching performed (the DynamicMethods and delegates are rebuilt on each call)

CachedReflector : With IL emission and caching (the resulting delegates are created only once, then accessed with a dictionary lookup)

I’m aware that the 2nd test case is ridiculous, you should never emit IL and generate methods at runtime and repeatedly, but I wanted to outline the importance of caching.

The serializer

When making this sample, I wanted to both provide a fast .NET reflection library as well as a proper generic implementation of a reflective serializer. But I didn’t want to spend time on string parsing/formatting, since serializers usually output a text file or a certain data format. So the tradeoff I chose is somewhat unusable in the real world…

It outputs objects which are a generalization tentative of all .NET objects. There are three main categories :

SerializedAtoms are indivisible, single-valued and immutable. All primitive types will serialize to atoms, in addition to strings, enums and nullable types.

SerializedCollections are multi-valued object bags that don’t give a specific meaning to keys or indices other than natural ordering. All classes that implement ICollection<T> will serialize into this.

SerializedAggregates are multi-valued object maps that use the key or index for indentification. All of which doesn’t fall in the two other categories will serialize to aggregates, so Dictionaries and just any other class.

Only atoms contain actual values, but it contains them as an object. There is no string conversion done in the end, it all remains in memory. Serialized objects also retain the name of their host field or dictionary entry if any, and the runtime type if different from the declared one.

To customize the serialization output to an extent, I made a custom attribute called [Serialization] which allows to force an alternate name to a serialized member, mark a member as ignored by the serializer, or mark it as required. I could’ve used “optional” instead, but I find it more logical to skip serialization of all null or default-valued fields.

Just like the XmlSerializer, it only serializes the public instance fields or properties. So unlike the BinaryFormatter (which is deep serialization), my serializer does shallow serialization.

I have tested the implementation with many (if not all) combinations of value-type/class, serialized object category and visibility, so I can say it’s pretty robust and tolerant on what you feed it.

Results

This is the whole point… how fast does “Fast .NET Reflection” go? Here are the timings for 10 outer loops (so 10 serializer creations) and 100 inner loops (100 serializations per outer loop), which means 1000 serializations or the same complex aggregate object.

Still, the highlight here is that by running the same serialization code with two different reflection function providers, using dynamic IL methods and a healthy dose of caching is eight (8!) times faster than using standard reflection.

Sample code

Even if you’re not interested in serialization, I suggest you take a look at the EmitHelper class and how it’s used in CachedReflector. All tasks that need Reflection in a time-critical context should use dynamic methods!

Recently I was coding some physics-enabled particle systems and ended up with some fairly CPU-intensive stuff that looped as times as there are active particles in a system, and as many as there are active particle systems, each update cycle.

And it got choppy.

I don’t like choppy.

So I had three choices :

Offload to the GPU using GPGPU or Vertex Textures, but doing that in XNA with Shader Model 2 support is… time-consuming. Especially when you have no background work on the subject.

Profile and optimize my physics code, or switch to a physics package like Havok or Farseer, but I expect that would also take too much time.

Multithread using loop parallelism to use 100% of my CPU power instead of 50%! (I have a Core 2 Duo processor, and there’s more and more mainstream PCs with dual-, quad- and even octo-cores)

I ended up doing the latter because it was the simplest, and multithreading is rarely ever simple. But in some specific cases, it’s not much of a headache, really.

If your looped operation does not change the context of further loops, that is if every loop can be done in any order and individually without any write locking, it takes like 30 minutes to get it to work in C#, and with no concurrency problems. It’s like offloading static Web content to a different server, just a matter of routing to the right hardware.

Annoyances and interrogations

ParameterizedThreadStart is not generic. If you have to pass a context to your threaded operation, you have to cast it back to what it really is, at each iteration. It’s unnecessary and annoying… is it slow?

Once a thread dies, you can’t resurrect it. Correct me if I’m wrong, but FSM knows I’ve tried, and a thread that finished its execution cannot be re-run, even if its thread-start method is the same. You have to instantiate another. Does that affect performance…?

The problem with thread creation is that I create a lot of temporary threads. They live a single update cycle, and I instantiate one per particle system per update cycle, at about 60 cycles per second. It sounds like it could slow things down.

Actually, this kinda makes sense if you’re calculating weights for a very wide Kaiser filter, which I will probably cover later. I had code laying around, which is why I used the elegantly-named Bessel function.

So, this is slow, because it’s an integral. And so multi-threading would be beneficial. The different strategies are :

Single-Threading; it’s always good to know how slow it went before all optimization… if only for programmer ego.

Multi-Threading, using the ParameterizedThreadStart delegate, which means we’ll have to cast the Object to our real context type.

Multi-Threading, using a class-local, strongly-typed context and the (parameterless) ThreadStart delegate. This way we avoid the cast, and sacrifice a weird variable in the host class’ header.

Multi-Threading, using a generic Thread wrapper that does the strongly-typed context caching instead of laying it around. It’s basically a proxy for Thread with the context variable.

Multi-Threading, using a single “kept-alive” Thread that is started once for all update cycles, and that is forced into an artificial idle state between update cycles.

I hoped, even half-expected that these solutions would go from slowest to fastest. The last one was particularly interesting because it skipped all the thread instantiations and instead, relies on a Monitor and a sync-lock object to “wait” between two update cycles. It also produces much less garbage.

Results

Here’s the result for 500 update cycles, in which I update a 500-sized data-set, in Release mode, out of the IDE so without any debugging, and as little stuff as possible running in the background. I used a Stopwatch object to calculate these timings.

The good news is, thread creation in .NET is very fast. No need to worry about that anymore. In fact, trying to circumvent that using a single kept-alive thread and monitor use makes it go almost 30% slower!

Also, casting the object context every iteration is faster than any other strongly-typed alternative. Unless the word “Object” in your code makes you want to shoot someone, it’s the best way to go. Kind of sad, that.

Sorry again for the absolute absence of comments, it’s late and I don’t feel like it. D:

A Note On Debugging

One thing that’s slightly alarming and certainly worth mentioning, is that the performance in the IDE (with Debugging, but not necessarily in “Debug” mode) is hugely different.

In debugging, the “kept-alive” thread is almost always faster than all other strategies, by as much as 15%. This never replicates in the real-world. So kids, always test out of the IDE, or add the empty green arrow (Start Without Debugging, Ctrl+F5) in your Visual Studio toolbar!